Chronic black crust/boogers under nose

[CatsMom]

My 4 month old Whiskey has the black crusty stuff that forms at the bottom of his nostrils. When we got him he just got over an upper respiratory infection. Our vet examined him and said he has a runny nose. He has no fever, no discolored mucus, and his health is fine. The vet said this could be a chronic runny nose that may never go away or he may outgrow it. We did put an antibiotic cream on the spots which didn't make a difference. We clean his nose with a damp cloth every morning and evening, yet the crusting still comes back. Anyone have any ideas to keep this stuff from coming back?